The principle of operation of a switched reluctance (SR) motor and its construction, energisation and control are well known with many publications such as that by P.J. Lawrenson et al "Variable-speed switched reluctance motors" IEE Proc B, Electr Power Appl., Vol 127, No 3, pp 253-265, 1980.
It is also well known that to obtain the best performance from an SR motor or generator the excitation of its phases should be carefully timed in relation to rotor position (see GB 1591346). This has previously necessitated the use of an incremental rotor position transducer which has generally been a disc with teeth or lines together with an optical or electromagnetic sensor which is able to detect the instants the teeth or lines cross prescribed positions.
The incorporation of a rotor position transducer on an SR motor creates additional electrical connections, additional cost and a potential source of unreliability. Various methods have therefore been proposed to eliminate the rotor position transducer. As an alternative it is possible to deduce rotor position by measurement and examination of the current and flux-linkage in one or more phases of the motor. This is commonly known as sensorless rotor position measurement. Since phase current generally needs to be measured in any case for control purposes and flux-linkage can be inexpensively obtained from measurement of phase voltage, sensorless rotor position measurement is commercially beneficial.
Sensorless methods generally depend on stored information of the flux-current-rotor position characteristics of the motor. A typical example of these characteristics is shown in FIG. 1. The storage of this data entails a two-dimensional array of significant size to achieve acceptable accuracy. Some methods are only appropriate for relatively low speed operation for which the well known `chopping` mode of current control applies and other methods are only appropriate for relatively high speed operation for which the well known `single-pulse` mode of current control applies.
A method which is more suited to lower speed operation that is known (for example, N M Mvungi and J M Stephenson "Accurate sensorless rotor position detection in an SR motor" EPE Conf Proc 1991 Vol I pps 390-393) involves the application of exploratory current pulses to each phase winding at periods during which the phase is not energised for torque production. To avoid a counter-productive torque these pulses need to be small in magnitude and, as a result, the measured flux is influenced by currents in other phases. This can cause error and corrections need to be made which require the additional two-dimensional storage of mutual magnetisation data.
A method by Hedlund (see WO91/02401) which is more suited to higher speed operation utilises the normal phase currents for position measurement purposes. However this method necessitates the continual sampling of flux and current and comparison of flux with stored values for a reference position. This may necessitate a dedicated digital processor to perform the sensorless position measurement.
An object of the present invention is to provide sensorless position measurement by sampling the current and flux in a phase only once per energisation cycle. This may enable the same digital processor which is used for the SR motor control to be used also for determining rotor position.